/*
 * Copyright (c) 2015, 2017, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 * questions.
 */
package com.nulldev.util.internal.backport.concurrency9;

import java.util.Map;
import java.util.Objects;

/**
 * An immutable container for a key and a value, suitable for use in creating
 * and populating {@code Map} instances.
 *
 * <p>
 * This is a <a href="package-summary.html#Value-based-Classes">value-based</a>
 * class; use of identity-sensitive operations (including reference equality
 * ({@code ==}), identity hash code, or synchronization) on instances of
 * {@code KeyValueHolder} may have unpredictable results and should be avoided.
 *
 * <p>
 * <b>API Note:</b><br>
 * This class is not public. Instances can be created using the
 * {@link Maps#entry Map.entry(k, v)} factory method, which is public.
 *
 * <p>
 * This class differs from AbstractMap.SimpleImmutableEntry in the following
 * ways: it is not serializable, it is final, and its key and value must be
 * non-null.
 *
 * @param <K> the key type
 * @param <V> the value type
 *
 * @see Maps#ofEntries Maps.ofEntries()
 * @since 9
 */
final class KeyValueHolder<K, V> implements Map.Entry<K, V> {
	final K key;
	final V value;

	KeyValueHolder(K k, V v) {
		key = Objects.requireNonNull(k);
		value = Objects.requireNonNull(v);
	}

	/**
	 * Gets the key from this holder.
	 *
	 * @return the key
	 */
	@Override
	public K getKey() {
		return key;
	}

	/**
	 * Gets the value from this holder.
	 *
	 * @return the value
	 */
	@Override
	public V getValue() {
		return value;
	}

	/**
	 * Throws {@link UnsupportedOperationException}.
	 *
	 * @param value ignored
	 * @return never returns normally
	 */
	@Override
	public V setValue(V value) {
		throw new UnsupportedOperationException("not supported");
	}

	/**
	 * Compares the specified object with this entry for equality. Returns
	 * {@code true} if the given object is also a map entry and the two entries'
	 * keys and values are equal. Note that key and value are non-null, so equals()
	 * can be called safely on them.
	 */
	@Override
	public boolean equals(Object o) {
		if (!(o instanceof Map.Entry))
			return false;
		Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
		return key.equals(e.getKey()) && value.equals(e.getValue());
	}

	/**
	 * Returns the hash code value for this map entry. The hash code is
	 * {@code key.hashCode() ^ value.hashCode()}. Note that key and value are
	 * non-null, so hashCode() can be called safely on them.
	 */
	@Override
	public int hashCode() {
		return key.hashCode() ^ value.hashCode();
	}

	/**
	 * Returns a String representation of this map entry. This implementation
	 * returns the string representation of this entry's key followed by the equals
	 * character ("{@code =}") followed by the string representation of this entry's
	 * value.
	 *
	 * @return a String representation of this map entry
	 */
	@Override
	public String toString() {
		return key + "=" + value;
	}
}
